1. Field of the Invention
This invention relates to a blank tube for constituting a main frame of a two-wheeled vehicle such as a bicycle, motorcycle or the like and more particularly to a main frame for a two-wheeled vehicle.
2. Description of the Prior Art
With a hitherto used main tube forming a main frame for a two-wheeled vehicle such as a bicycle, the main tube has been formed as a member separate from a head tube and a seat tube. In order to join respective tube members consisting the main frame, respective ends of the tube members have been inserted into and welded to separate lugs arranged between the tube members or lugs have been cast the ends of the tube members to form a unitary main frame.
The joints of a tube member at its ends are in general subjected to the maximum stresses in the member. Accordingly, if the welding is incomplete resulting, for example, from unskillful welding operation, extreme irregularity in strength at the weld will cause stress concentration. Moreover, the tube members are heated at the joints by welding or casting, so that the quality of the material of the tube members changes by thermal attack to lower its strength. Furthermore, such a joining operation is so complicated that its production efficiency is low.
In order to pass brake cables or electric lead wires in the main tube, the front and rear ends of the main tube are formed with hemiconical protrusions welded thereto and then formed at the protrusions with through-apertures obliquely passing through the main tube.
However, such a method requires many members for the operation and is poor in production efficiency because of its complicated working and assembling operation. Moreover, the heating in welding is apt to cause local weakness in the tube.
A main tube having a circular cross-section is often anxious for its strength because of a great force acting on the main tube in its axial direction. Accordingly, a blank tube having sufficient strength without increasing its weight has been expected.